This invention relates to the general art of extruding thermoplastic material in a sheet form, then directly feeding and forming this hot plastic sheet material into hollow objects, such as food containers.
Such processes in general require a device for extruding the thermoplastic in sheet form, a set of temperature controlled tempering rolls to control the thickness of the sheet web and to reduce its overall temperature to the desired forming temperature, a device to transport the molten, often sagging web into a forming machine and a device to trim out the finished parts from the web, after these are formed and stabilized.
The range of materials which can be extruded in sheet form and used in the method of invention generally includes almost the whole known range of thermoplastics, and combinations thereof, either blended or simultaneously co-extruded in discrete layers by multiple extruders feeding into a single die.
In the application of this invention we are particularly, but not exclusively, interested in those materials which have melts which behave more as viscous fluids than as rubbery membranes. Typically, crystalline polyolefines such as High Density Polyethylene and Polypropylene have a sharply defined melting point and a melt rheology resembling a highly viscous fluid. Such materials sustain stress primarily by viscous resistance, and therefore sag or creep, when suspended as a sheet without full support. The cohesive elasticity which materials such as PVC and Polystyrene exhibit in their molten state, make them relatively easy to feed into thermoforming equipment and these have traditionally been the preferred materials for thermoforming.
Recently however, the advent of the high oxygen barrier polymers such as Ethylene Vinyl Alcohol (EVOH) Polyvinylidene Chloride (PVDC), has given rise to a new class of food packaging, wherein food is packed and sealed into a plastic package and sterilized in steam retorts, in much the same way as metal cans, at retorting temperatures up to 140.degree. C. Polypropylene is one of the few readily available resins with the relatively high temperature resistance necessary to withstand steam serialization. It is often combined, usually by co-extrusion, with a layer of the aforementioned high barrier plastic, to produce the base material for high barrier plastic packaging.
Other forms of fabrication, such as injection molding, are not suited to economic production of multilayer hollow containers and the relatively mature art of thermoforming is therefore currently undergoing a developmental transformation aimed to achieving economic means of thermoforming retortable polypropylene-based high-barrier containers.
We have found that it is difficult to reheat and then thermoform pre-extruded polypropylene sheet. The melt sag which occurs immediately after the material passes through its crystalline melting point makes it very difficult to heat a suitably sized area of suspended sheet by known means, such as infrared radiation, and then to feed such a sheet of sagging molten material into a thermoforming machine of normal commercial size and output. Many of the current thermoforming process operators have found that they can often achieve reasonable results by forming polypropylene just below its crystalline melting point. This so-called solid phase forming usually leaves residual stress in the walls of a finished container and results in unsightly distortion when this stress is released during sterilization. A further problem of solid-phase forming is that the melting points of the commonly used EVOH and PVDC resins are higher than that of polypropylene, and a thin barrier layer of this material can be relatively easily damaged during forming at the solid-phase forming temperature of polypropylene.
Hence, we have developed a process and method by which polypropylene may be transported and formed in the so-called melt phase. Extruding the melt directly into a forming process overcomes many of the difficulties of reheating premade polypropylene sheet and lends further economies, such as a saving of energy which would otherwise be expended in reheating the premade sheet.
Two types of forming or "Thermoforming" machines can be used for such extrusion fed processes: Continuous and Intermittent Thermoformers. Continuous thermoformers, typically as described in Kurz, U.S. Pat. No. 4,235,579, operate with forming tools which move in approximate synchronization with continuously delivered molten sheet. In such machines, the problem of transporting the molten web is often solved by developing relatively constant tension in the web by running the moving forming tooling at a faster speed than the speed at which the molten web leaves the tempering rollers. Such a method is described in Flecknoe-Brown, U.S. patient application Ser. No. 762,069, herein incorporated by reference.
Intermittent thermoformers require feeding of the web in discrete lengths. When a direct extrusion feeding method is used to supply such machines, means must be provided to compensate for the lag caused between the continuous extrusion of the web and the intermittent feeding of discrete lengths of this continuously generated web, to the forming machines.
Thiel, U.S. Pat. No. 4,105,386 teaches the use of tempering rolls to form cooled supporting layers on an extruded web, and a movable compensating or "dancing" roller which is moved to accumulate the extra length of web between intermittent feeding.
There are a number of other thermoplastic materials which have fluid melts like the polyolefines. These include the polyalkylene terepthalates, polycarbonates and polyamides. These materials all have desirable properties for formed parts but are notoriously difficult, if not impossible, to feed into thermoformers without some means of supporting the soft, sagging, sheet of melt. In the past, attempts to use driven conveyor belts for supporting and transporting such materials for loading into a thermoformer have been frustrated by the natural tendency of these fluid melt materials to wet the belt material, and to remain adhered to it.
The use of a belt conveyor to support a molten plastic extrudate is not new in principle, being described in Loosen, European patent application Ser. No. 0,226,748 and in Asano, U.S. Pat. No. 4,459,093, which are herein incorporated by reference. This prior art does not address the problem of dealing with the belt adhesion of fluid melt materials.
Yet other methods disclosed in the prior art include the moving of the entire extruder towards and away from the forming station (Asano, U.S. 4,150,930) in combination with a synchronized extending carrier, comprising side chains and clamps which hold and support the sides of the molten sheet and convey it into the forming station. There is an obvious mechanical difficulty in moving the relatively massive extruder quickly enough to keep up with a thermoformer which may typically operate at 10 to 20 strokes per minute. There is also an evident further defect in that there is insufficient transverse support for sheet having a fluid melt, when suspended between longitudinal edge supporting clamps, only.
Finally, another approach is described in Keifer, Federal Republic of Germany Patent No. 2,634,976 wherein a catenary of molten web is supported between two driven rollers, initially held widely apart, then brought closer together to allow a festoon of material to develop between the two rollers, so taking up the excess length of material between feeds, whilst also ensuring that a zone of chilled material does not develop by constant contact between the downstream rollers and the molten sheet, held stationary between feeds. This approach again does not provide adequate support for a web of soft, fluid melt material, such as a molten polyolefine.